The present invention relates to the preparation and use of certain prostanoic acid analogs and more specifically to novel azaprostanoic acids and their use, inter alia, as inhibitors of blood platelet aggregation through selective antagonism of the activity of prostaglandin H.sub.2 (PGH.sub.2) and thromboxane A.sub.2 (TXA.sub.2).
In the recent past an enormous amount of research effort has had as its focus the elucidation of the biological properties and activities of blood platelets. In circulation platelets do not normally adhere to each other or to intact blood vessel endothelium but can adhere and spread to non-endothelial surfaces, (e.g., subendothelial collagen at the site of vascular injury) aggregate in response to a variety of stimuli and secrete substances which both cause further aggregation and mediate other biological responses. The so-called "reversible" or "first phase" platelet aggregation may be initiated by the presence of adenosine diphosphate (ADP) and is characterized by clumping of the platelets. "Second phase" or "irreversible" aggregation is marked by platelet synthesis of prostaglandin endoperoxides and TXA.sub.2 with subsequent secretion of ADP, serotonin, calcium, and other materials.
Numerous attempts have been made to provide therapeutic agents which would modulate the sequence of biochemical events leading up to platelet aggregation at the site of exposure of subendothelium connective tissue and/or systemically upon the triggering of aggregation processes by substances in the circulatory system. Agents capable of control of the biochemical pathways leading to aggregation would have therapeutic potential in the prevention or treatment of myocardial infarction, myocardial ischemia, pulmonary thromboembolism, disseminated intravascular coagulation, and circulatory complications arising from extracorporeal circulation, oral contraceptive therapy, rheumatic fever, congestive heart failure and the like. See, generally, Weiss, H. J., N.E. Jour. Med., 298 Nos. 24 & 25, pp. 1344-1347, 1403-1406 (1978).
The therapeutic potential of any of the so-called "antiplatelet drugs" must be determined within the context of its specific effects upon the "arachidonic acid cascade" of biochemical reactions leading up to aggregation of platelets during the irreversible phase of aggregation.
Table I below sets forth a simplified schematic representation of the present knowledge of principle biochemical and physiological "events" in the arachidonic acid cascade and metabolism of arachidonic acid within the platelet.
TABLE I ______________________________________ Activating agents ##STR2##
As noted in Table I, a large number of substances are capable of acting as activating agents in platelet aggregation: ADP; collagen; arachidonic acid; thrombin; serotonin; and epinephrine. Put most simply, arachidonic acid is converted by cyclooxygenase to a series of endoperoxides, PGG.sub.2 and PGH.sub.2. The endoperoxides are, in turn, subjected to conversion by thromboxane synthetase within the platelet to thromboxane A.sub.2. PGH.sub.2 and/or TXA.sub.2 are believed to operate upon a receptor substance to effect platelet aggregation and vasoconstriction. Essentially simultaneously, endoperoxides can also undergo transformation into the prostaglandins PGD.sub.2, PGE.sub.2 and PGF.sub.2.alpha.. More significantly, the endoperoxides are precursors for the formation of the potent inhibitor of platelet aggregation, prostacyclin (PGI.sub.2). Current evidence leads to the conclusion that during vascular injury platelets sticking to the damaged area "feed" endoperoxides to prostacyclin synthetase in the blood vessel wall adjacent the damaged area, thereby preventing occlusion of the vessel by maintaining a proper balance between a pro-aggregatory agent, TXA.sub.2, and an anti-aggregatory agent, PGI.sub.2. The presumed mechanism of action of PGI.sub.2 resides in its capacity to stimulate an elevation of cyclic adenosine monophosphate (cAMP) levels, while TXA.sub.2 is thought to act by depressing cAMP formation.
Although many agents are known to inhibit platelet aggregation in vitro, no effective and benign antithrombotic agent has yet been found. Nonsteroidal anti-inflammatory drugs such as aspirin and indomethacin or the uricosuric agent sulfinpyrazone are known inhibitors of prostaglandin biosynthesis and platelet secretion and have been tested extensively as antithrombotic agents, with controversial results. Aspirin and indomethacin both inhibit prostaglandin biosynthesis by blocking the conversion of substrate fatty acids to endoperoxides by the cyclooxygenase enzyme. Aspirin has been shown to specifically and irreversibly acetylate the active site of the cyclooxygenase. After a single oral dose of aspirin, inhibition of collagen-induced aggregation persists as long as 4 to 7 days, which is the half-life of the platelet.
Aspirin is known to prolong bleeding times and can severely aggrevate the condition of patients with bleeding disorders. However, some success has been reported for the use of aspirin and sulfinpyrazone in the treatment of arterial thrombi, atherosclerosis, and in inhibiting thrombus formation in patients with prosthetic heart valves, as well as in extracorporeal shunts connected to experimental animals. Nevertheless, most aspirin studies have been retrospective with inappropriate controls.
From the point of view of the arachidonic acid cascade in platelets and the possible role of thromboxanes and prostacyclin in hemostasis, the major criticism in the use of aspirin and other non-steroidal anti-inflammatory drugs is their indiscriminant inhibition of this balanced hemostatic control. Thus, by inhibition of the cyclooxygenase enzyme in both platelet and blood vessel wall, the synthesis of both pro-aggregatory (TXA.sub.2) and anti-aggregatory (PGI.sub.2) agents are blocked. A more rational approach to this problem is acknowledged to be the design of compounds that selectively modulate the various biosynthetic pathways after endoperoxide formation.
Several of the primary prostaglandins, PGE.sub.1 and PGD.sub.2, appear to be likely candidates as antithrombotic agents. However, their very short biological half-lives preclude their use by oral administration, and must therefore be administered by continuous infusion. Another obvious drawback in the use of primary prostaglandins is their multitude of other physiological activities, primarily stimulation of smooth muscle and gastric secretion, as well as their effects on blood pressure and renal blood flow.
Drugs which selectively inhibit TXA.sub.2 formation would be theoretically preferable as antithrombotic drugs since generation of prostacyclin would remain unimpaired. Several compounds are known to selectively inhibit thromboxane synthetase without significantly blocking the cyclooxygenase.
Benzydamine, an anti-inflammatory agent, is more than twice as effective in inhibiting thromboxane synthetase than in inhibiting prostaglandin biosynthesis (cyclooxygenase). Imidazole, selectively inhibits thromboxane synthetase without inducing changes in platelet cAMP levels.
Gorman et al., [P.N.A.S., 74, p. 4007 (1977)] synthesized an azo prostanoic acid analog ##STR3## which proved to be a potent inhibitor of thromboxane synthetase, as well as an inhibitor of platelet aggregation induced by PGH.sub.2, arachidonic acid, or collagen, and of the second wave of ADP or epinephrine-induced aggregation. Their data also suggest that the azo analog is a competitive inhibitor of PGH.sub.2 binding to the thromboxane synthetase enzyme. Inhibition of TX synthetase by the compound led to an increase in PGE.sub.2, indicating that the cyclooxygenase enzyme was not affected. In a further study, imidazole and the azo analog have been found to selectively inhibit thromboxane synthetase in a concentration dependent fashion, in both platelet rich plasma and washed platelet suspensions. The azo analog suppressed the attendant aggregation induced by arachidonate in a parallel, concentration-dependent fashion, whereas the influence of imidazole on aggregation was erratic. However, both inhibitors exhibited consistent behavior during aggregation of platelet rich plasma when induced by PGH.sub.2. Several important characteristics of these inhibitors could, however, seriously limit their potential use as effective antithrombotic agents. In this respect imidazole appears to also possess direct agonist activity which may explain its ability to potentiate aggregation in washed platelets. In addition, high concentration of imidazole, i.e., 2 mM, are necessary to achieve inhibition of thromboxane synthetase. Furthermore, even though the above-noted azo analog is far more potent in blocking TXA.sub.2 synthesis, recent evidence suggests that inhibitory activity may also extend to prostacyclin synthetase. Such an affect could seriously diminish in vivo antithrombotic activity since, as previously mentioned, PGI.sub.2 is thought to plate a significant role in reducing platelet reactivity.
While specific inhibition of thromboxane synthetase appears to be a potential route to the development of anti-thrombotic agents, the same effect should be accomplished by selectively antagonizing the action of TXA.sub.2 at the receptor level, and this would have the advantage of allowing the entire endoperoxide-thromboxane-prostacyclin system to remain in balance, without the shunting of endoperoxides to unwanted side products. To date, no reports of this approach have appeared in the literature. There exists, therefore, a need in the art for stable, biologically active substances which will selectively antagonize the activity of TXA.sub.2 within the arachidonic acid cascade.